This invention relates to an automatic exposure time control device for use in a single lens reflex cameras, more particularly to such control device suitable for use in single lens reflex cameras having a TTL light measuring system.
With single lens reflex cameras of the TTL light measuring type, the rays coming from the object to be photographed and incident on the photocell is blocked by the reflecting mirror which is swung up after the shutter is released, so that for automatic exposure time control, there is the necessity of storing the information of the brightness of the object before the mirror is swung up. A system is known for controlling the exposure time with accuracy in response to widely varying intensities of the light from the object. According to this system, a voltage proportional to the logarithm of the intensity of light from the object is stored in a capacitor before the reflecting mirror is swung up and, for the commencement of exposure, the logarithmically converted voltage is converted to a current in proportion to the light intensity by antilogarithmic conversion which current is charged in another capacitor. When the voltage of the capacitor reaches a specified value, the shutter is closed. This system requires a high input impedance element as an input circuit subsequent to the memory capatitor. A field effect transistor is usable as such high input impedance element, but it involves great drift due to temperature and therefore has the drawback of necessitating a circuit for compensating for the drift. Supposedly, another type of high impedance circuit is usable, but the circuit still has the drawback of requiring a high power source voltage. In order to overcome these drawbacks, the present inventor et al. have already proposed an automatic exposure time control device as disclosed in U.S. Pat. application Ser. No. 429,391. This device is illustrated in FIG. 1. With reference to the Figure, a trigger switch 8' is provided between the negative terminal of a power source E and the emitter of an antilogarithmic conversion transistor 4 connected to the output terminal of a light measuring circuit 1 which responds, in the known manner, to the light traversing the camera objective lens L and diaphragm A. The switch 8' is closable with the initiation of movement of a shutter opening member after the shutter has been tripped and is openable in relation to completion of film winding operation. The system further includes a switch 7' disposed between the trigger switch 8' and a memory capacitor 2 on a lead wire connected to the negative terminal of the power source E. The switch 7' is closed at the initial stage of operation of the shutter button and is opened upon the completing of movement of a shutter closing member. The switch 7' also serves as a power source switch for a control circuit 9, an electromagnet 10 and the transistor 4. As will be apparent from the above description, the switch 8' is always open before the shutter is opened, so that the potential stored in the memory capacitor 2 will not leak from between the base and the emitter of the transistor. Thus it is possible to use a usual transistor as the transistor 4. However, when the travel of the shutter is initiated with the switch 7' closed after the shutter button is depressed, the whole current passing through the transistor 4 and the control circuit 9 passes through the switch 7', resulting in a large contact potential difference at the switch 7' and consequently leading to an error in the exposure time. More specifically, the contact resistance of the switch 7' varies within the approximate range of 100 to 300 m.OMEGA. and, if a current of 15 mA passes through the switch 7' with the initiation of travel of the shutter, the voltage drop occurring between the opposite terminals of the switch 7' is about 3 mV on average. As is well known, when the voltage between the base and the emitter of transistor changes by about 18 mV at ordinary temperature (25.degree.C), the resulting changes in the current between the base and the emitter and in the collector voltage are two-fold. Accordingly, since 18 mV corresponds to 1 EV, the voltage drop due to the resistance of the switch 7' gives to the exposure an influence which is numerically 0.17 EV on average. This influence is not negligible.